3.9. Typing Callable

  • Before Python 3.9 you need from typing import List, Set, Tuple, Dict

  • Since Python 3.9: PEP 585 -- Type Hinting Generics In Standard Collections

3.9.1. Return

>>> def say_hello() -> str:
...     return 'My name... José Jiménez'

3.9.2. Parameters

Required:

>>> def add(a: int, b: int):
...     return a + b

Optional:

>>> def add(a: int = 1, b: int = 1):
...     return a + b

3.9.3. Union

  • Since Python 3.10: PEP 604 -- Allow writing union types as X | Y

>>> def add(a: int | float, b: int | float) -> int | float:
...     return a + b

3.9.4. Optional

>>> def find(character, text) -> int | None:
...     position = text.find(character)
...     if position == -1:
...         return None
...     return position

3.9.5. Alias

  • Since Python 3.10: PEP 604 -- Allow writing union types as X | Y

>>> number = int | float
>>>
>>> def add(a: number, b: number) -> number:
...     return a + b

3.9.6. Exception

>>> def div(a: float, b: float) -> float:
...     if b == 0:
...         raise ZeroDivisionError
...     return a / b

>>> def div(a: float, b: float) -> float | Exception:
...     if b == 0:
...         raise ZeroDivisionError
...     return a / b

>>> def div(a: float, b: float) -> float | ZeroDivisionError:
...     if b == 0:
...         raise ZeroDivisionError
...     return a / b

3.9.7. Literal

SetUp:

>>> from typing import Literal

Definition:

>>> def open(filename: str, mode: Literal['r','w','a']) -> None:
...     pass

Usage:

>>> open('data.csv', mode='w')  # mypy: OK
>>> open('data.csv', mode='r')  # mypy: OK
>>> open('data.csv', mode='a')  # mypy: OK
>>> open('data.csv', mode='x')  # mypy: ERROR

3.9.8. Callable

SetUp:

>>> from typing import Callable

Define:

>>> def add(a: int, b: int) -> int:
...     return a + b
>>>
>>> x: Callable = add
>>> x: Callable[..., int] = add
>>> x: Callable[[int,int], int] = add

Parameter:

>>> def run(func: Callable[[int, int], float]):
...     ...

3.9.9. Iterator

  • All Generators are Iterators

  • Generator[yield_type, send_type, return_type]

  • Iterator[yield_type]

SetUp:

>>> from typing import Iterator, Generator

Generator type annotations:

>>> def fib(n: int) -> Generator[int, None, None]:
...     a, b = 0, 1
...     while a < n:
...         yield a
...         a, b = b, a + b

All Generators are Iterators so you can write:

>>> def fib(n: int) -> Iterator[int]:
...     a, b = 0, 1
...     while a < n:
...         yield a
...         a, b = b, a + b

3.9.10. Annotations

>>> def add(a: int, b: int) -> int:
...     return a + b
>>>
>>>
>>> add.__annotations__
{'a': <class 'int'>, 'b': <class 'int'>, 'return': <class 'int'>}

3.9.11. Errors

  • Python will execute without even warning

  • Your IDE and mypy et. al. will yield errors

>>> def add(a: int, b: int) -> int:
...     return a + b
>>>
>>>
>>> add('Mark', 'Watney')
'MarkWatney'

3.9.12. Good Engineering Practices

>>> def add(a: int | float,
...         b: int | float,
...         ) -> int | float:
...     return a + b

3.9.13. Literal String

  • Since Python 3.11: PEP 675 -- Arbitrary Literal String Type

SetUp:

>>> from typing import LiteralString

Example:

>>> def execute(sql: LiteralString):
...    ...
>>>
>>> username = 'mwatney'
>>>
>>>
>>> execute('SELECT * FROM users WHERE login="mwatney"')                  # ok
>>> execute('SELECT * FROM users WHERE login=' + username)                # ok
>>> execute('SELECT * FROM users WHERE login=%s' % username)             # error
>>> execute('SELECT * FROM users WHERE login=%(login)s' % {'login': username}) # error
>>> execute('SELECT * FROM users WHERE login={}'.format(username))       # error
>>> execute('SELECT * FROM users WHERE login={0}'.format(username))      # error
>>> execute('SELECT * FROM users WHERE login={login}'.format(login=username))  # error
>>> execute(f'SELECT * FROM users WHERE login={username}')                # error

3.9.14. Use Case - 0x01

>>> def valid_email(email: str) -> str | Exception:
...     if '@' in email:
...         return email
...     else:
...         raise ValueError('Invalid Email')
>>>
>>>
>>> valid_email('mwatney@nasa.gov')
'mwatney@nasa.gov'
>>>
>>> valid_email('mwatney_at_nasa.gov')
Traceback (most recent call last):
ValueError: Invalid Email

3.9.15. Use Case - 0x02

>>> def find(text: str, what: str) -> int | None:
...     position = text.find(what)
...     if position == -1:
...         return None
...     else:
...         return position
>>>
>>>
>>> find('Python', 'x')
>>> find('Python', 'o')
4

3.9.16. Use Case - 0x03

>>> from urllib.request import urlopen
>>> from typing import Any
>>>
>>>
>>> def fetch(url: str,
...           on_success: Callable[[str], Any] = lambda result: ...,
...           on_error: Callable[[Exception], Any] = lambda error: ...,
...           ) -> None:
...     try:
...         result: str = urlopen(url).read().decode('utf-8')
...     except Exception as err:
...         on_error(err)
...     else:
...         on_success(result)

>>> def handle_result(result: str) -> None:
...     print('Success', result)
>>>
>>> def handle_error(error: Exception) -> None:
...     print('Error', error)
>>>
>>>
>>> fetch(
...     url='https://python3.info',
...     on_success=handle_result,
...     on_error=handle_error,
... )  

>>> fetch(
...     url='https://python3.info',
...     on_success=lambda result: print(result),
...     on_error=lambda error: print(error),
... )

3.9.17. Use Case - 0x04

>>> import json
>>> from datetime import date
>>> from typing import Any

>>> data = {'firstname': 'Mark', 'lastname': 'Watney'}
>>> json.dumps(data)
'{"firstname": "Mark", "lastname": "Watney"}'

>>> data = {'firstname': 'Mark', 'lastname': 'Watney', 'birthdate': date(1969, 7, 21)}
>>> json.dumps(data)
Traceback (most recent call last):
TypeError: Object of type date is not JSON serializable

>>> def encoder(obj: Any) -> str:
...     if isinstance(obj, date):
...         return obj.isoformat()
...
>>>
>>> json.dumps(data, default=encoder)
'{"firstname": "Mark", "lastname": "Watney", "birthdate": "1969-07-21"}'

3.9.18. Further Reading